add filename extension mapping for mkv files on OSX

My Mkv files show up as "other" in the system type files, so when searching for movies, or organizing files as types, thy get slotted in with all the stuff the system does not have any type information for.

From what I understand its up to the default application to provide type information to the system.

http://superuser.com/questions/371892/osx-assign-extension-to-content-kind